1. Field of the Invention
The present invention relates to a method and apparatus for inputting position information such as coordinate data, particularly for use with a large screen display system. In detail, the present invention relates to a position information input apparatus and method for generating position information such as a coordinate expressing a position designated on a display screen by an operator. The operator may use a designation device for designating the position. The input position information can be used for controlling an externally connected computer, or for writing characters and graphics.
2. Description of Related Art
Some of the prior coordinate input devices sense a light spot on the screen by using a CCD area sensor or linear sensor. The devices calculate and output coordinate values by performing an image processing using centroid coordinates, pattern matching, etc. Some of the prior coordinate input devices use position detection elements (an analogue device for obtaining an output voltage corresponding to a position of a light spot) called PSDs (Position Sensitive Detector).
For example, Japanese Patent Publication No. 7-76902 discloses an apparatus which senses a light spot of a parallel beam of visible light using a video camera, and calculates coordinate values. The apparatus simultaneously sends and receives a control signal by infrared diffusion light. Japanese Laid-Open Patent Application No. 6-274266 discloses an apparatus which detects a coordinate value by a linear CCD sensor and a special optical mask. On the other hand, Japanese Patent No. 2503182 discloses a structure of an apparatus using PSD and a correction method for outputting a coordinate value from the apparatus.
Nowadays, the brightness of large screen displays has improved, they can be used satisfactorily even in a brightly illuminated environment, and the display is expected to be in great demand. A coordinate input apparatus must be more tolerant to ambient light for use in such an environment together with the large screen display. The use of devices using an infrared light as a wireless communication unit is increasing recently. Since levels of infrared light and visible light are increasing in ambient light, it is an important feature for the apparatus to have a high tolerance to ambient light.
However, as is disclosed in Japanese Patent Publication No. 7-76902 and Japanese Laid-Open Patent Application No. 6-274266, ambient light is only controlled by an optical filter in the apparatus using a prior-art CCD sensor. To the contrary, as is disclosed in Japanese Patent No. 2503182, an effect of the ambient light is controlled by synchronized detection of frequency-modulated light intensity in the apparatus using a PSD. The apparatus with an optical filter achieves higher tolerance to ambient light.
The brightness and resolution of large screen displays are increasing. The resolving power in a coordinate input apparatus must also be increased, but this is difficult to achieve in an apparatus using a PSD having a high tolerance to ambient light. This is because a dynamic range of sensor output voltage corresponds to a range of input area exactly, then the apparatus must have at least 60 db or higher S/N ratio to achieve a resolving power or detection accuracy of 1 mm. As is disclosed in Japanese Patent No. 2503182, since a digital correction of linear error is necessary, a high precision analogue circuit, a multi bit AD converter and an arithmetic circuit are required. Since the S/N ratio of the sensor output signal depends on luminous energy and sharpness of the light spot, not only control of the ambient light but also a bright high precision optical system are required for producing a light spot. Accordingly, the apparatus increases in cost and size.
However, Japanese Patent Publication No. 7-76902 discloses a method using a plurality of video cameras simultaneously as a method for increasing a resolving power by using a CCD sensor. Such a method makes an apparatus increase in cost and size, and if a video camera having many pixels is used in the apparatus, it increases the apparatus' cost and size more than using a plurality of video cameras, and an artificially high resolving power realized in real time operation by performing an image processing also increases the apparatus' cost and size because high speed image processing is required.
Japanese Laid-Open Patent Application No. 6-274266 discloses a high resolving power realized by using a special optical mask and signal processing. The resolving power can be increased if the ambient light is weak and high S/N ratio is ensured. In practice, a linear sensor images a line, and the ambient light has more effect on a line image in this situation because the line image can't be separated from ambient light within a plane, in contrast to a situation in which an area sensor images dots. For this reason, the apparatus is able to be used only in a special environment having a low level of ambient light.
Generally, the optical elements like a CCD, etc. are mass produced by semi-conductor processes, but the various features such as optic-electric conversion efficiency, etc. differ widely depending on the product environment, the product condition, product lot, etc. For example, a difference of about plus and minus 30% in the optic-electric conversion efficiency is possible as a parts tolerance. When a CCD is worked at an electric source voltage of 5V, then if the nominal value of CCD output voltage in a dark condition is about 1.5V, the actual value for a particular device may be from about 1.0 to 2.0 V (about plus and minus 30%) due to the acceptable parts tolerance. Therefore, the threshold value must be set at 2.0V or more when determination of a light output is obtained by comparing the output with a fixed threshold voltage, but elements at the limits of the tolerance range will be unable to detect satisfactorily and the yield is thus decreased, in a structure which receives only low levels of light (namely CCD output is small). To adjust individually the threshold value set in each CCD described above can raise the light sensitivity, but assembly steps are increased and the cost goes up since an adjustment assembly is required.
The present invention aims to solve the above problems of the prior art, and one aspect of this invention is to control the effect of the ambient light and to provide a coordinate input apparatus and method having a high resolving power and a high precision.
Another aspect of this invention is to provide a small size and low cost coordinate input apparatus and method.